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| United States Patent |
5,243,170
|
|
Maruyama
, et al.
|
September 7, 1993
|
Method for deposition of hexagonal diamond using hydrogen plasma jet
Abstract
An improved method is proposed for providing a coating film of diamond on
the surface of, for example, a cutting tool as the substrate by the plasma
jet deposition method, in which the deposited diamond film has, different
from the cubic crystalline structure formed under conventional conditions,
a predominantly hexagonal crystalline structure so as to greatly enhance
the advantages obtained by the diamond coating of the tool in respect of
the hardness and smoothness of the coated surface. The improvement
comprises: using hydrogen alone as the plasma-generating gas; controlling
the pressure of the plasma atmosphere not to exceed 300 Torr; keeping the
substrate surface at a temperature of 800.degree.-1200.degree. C.; and
making a temperature gradient of at least 13,000.degree. C./cm within the
boundary layer on the substrate surface.
| Inventors:
|
Maruyama; Katsuhisa (Tsukuba, JP);
Makino; Mitsuo (Tsukuba, JP);
Kikukawa; Nobuyuki (Tsukuba, JP);
Shiraishi; Minoru (Kawasaki, JP)
|
| Assignee:
|
Agency of Industrial Science and Technology (Tokyo, JP)
|
| Appl. No.:
|
660034 |
| Filed:
|
February 26, 1991 |
Foreign Application Priority Data
| Current U.S. Class: |
219/121.59; 219/121.47; 219/121.51; 427/122; 427/249.8; 427/446 |
| Intern'l Class: |
B23K 009/00 |
| Field of Search: |
219/121.59,121.47,121.51,75,76.16
427/34,39,122,249
118/723
428/457,408
|
References Cited
U.S. Patent Documents
| 4842945 | Jun., 1989 | Ito et al. | 428/457.
|
| 4919974 | Apr., 1990 | McCune et al. | 427/249.
|
| 4992082 | Feb., 1991 | Drawl et al. | 428/249.
|
| 5052339 | Oct., 1991 | Vakerlis et al. | 118/723.
|
| 5096736 | Mar., 1992 | Anthony et al. | 427/39.
|
Primary Examiner: Paschall; Mark H.
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Claims
What is claimed is:
1. In a plasma jet method for the deposition of a coating film of diamond
having an at least partly hexagonal crystalline structure by blowing a
plasma jet of a plasma-generating gas containing a gaseous carbon source
compound at the surface of a substrate in a gaseous atmosphere of reduced
pressure, the improvement which comprises conducting said deposition under
the following conditions:
(a) using hydrogen alone as the plasma-generating gas;
(b) keeping the pressure of the gaseous atmosphere in the range from 1 Torr
to 300 Torr;
(c) keeping the surface of the substrate at a temperature in the range of
from 800.degree. C. to 1200.degree. C.; and
(d) making the temperature gradient of at least 13,000.degree. C./cm within
the boundary layer on and in the direction perpendicular to the substrate
surface.
2. The improvement as claimed in claim 1 which further comprises:
(e) introducing the gaseous carbon source compound into the plasma jet of
hydrogen in a direction perpendicular to the stream of the plasma jet.
3. The improvement as claimed in claim 2 in which the gaseous carbon source
compound is methane.
4. A diamond film deposited on the surface of a substrate having an at
least partly hexagonal crystalline structure characterized by the Raman
spectra at wave numbers of about 1140 cm.sup.-1 and 1470 cm.sup.-1, the
film being deposited by the plasma jet method which comprises conducting
the deposition under the following conditions:
(a) using hydrogen alone as the plasma-generating gas;
(b) keeping the pressure of the gaseous atmosphere in the range from 1 Torr
to 300 Torr;
(c) keeping the surface of the substrate at a temperature in the range of
from 800.degree. C. to 1200.degree. C.; and
(d) making a temperature gradient of at least 13,000.degree. C./cm within
the boundary layer on and in the direction perpendicular to the substrate
surface.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method for the deposition of a film of
hexagonal diamond on the surface of a substrate or, more particularly, to
a method for the deposition of a film of hexagonal diamond by utilizing a
hydrogen plasma jet.
As is well known, it is a recently developed technology that various kinds
of cutting tools and the like for working of extremely hard materials are
provided with a surface coating film of diamond in an object to improve
the precision of mechanical working therewith and to increase the
durability of the tool edge. Such a coating film is deposited on the
substrate surface as a thin layer of diamond having a crystallographically
cubic structure, for example, by the plasma jet method and apparatus
disclosed in Japanese Patent Kokai 1-215795 and 1-157496, according to
which a plasma jet is generated by ejecting a gaseous mixture of carbon
monoxide or methane as the carbon source and hydrogen and/or argon out of
an anode nozzle of a special design into an arc formed by electric
discharge and blown at the surface of a substrate so that a film of
diamond is deposited thereon. Although the coating film of cubic diamond
can be deposited in a considerably high efficiency in this known method, a
problem therein is that the thus deposited diamond film consists of
polycrystals of relatively coarse diamond crystallites having a diameter
of 20 .mu.m or larger so that the improvement in the cutting tool obtained
thereby in the precision of mechanical working with the tool cannot be as
high as desired and nicking of the tool edges would readily occur to
decrease the durability of the tool. An alternative method is known for
the deposition of a coating film of diamond by which the size of the
diamond crystallites can be much smaller, though not small enough, but
this method can hardly be used in practical applications because of the
unduly low velocity of deposition on the substrate surface. Japanese
Patent Kokai 63-99138 discloses a plasma CVD method for forming a hard
carbonaceous coating film consisting of graphite and hexagonal diamond on
the surface of a machining tool, in which the substrate at a high negative
potential is held apart from the plasma-generating electrodes to produce
plasma of ethylene. Although some lubricity can be imparted to the
surface, the thus coated tool is not always quite satisfactory in respect
of the durability because the carbonaceous coating film thus formed
consists mainly of graphite with a relatively minor fraction of hexagonal
diamond.
The inventors accordingly have continued extensive investigations to
develop an efficient method for the deposition of a coating film of
diamond on the surface of a substrate without the above described problems
and disadvantages in the prior art methods. In the course of their
investigations, the inventors have found that the diamond film as
deposited by the plasma jet method sometimes has an at least partly
hexagonal crystalline structure and got an idea to seek a possibility of
obtaining a quite satisfactory coating film of diamond by utilizing fully
developed hexagonal phase of diamond which is a metastable phase of
diamond quite different from the ordinary cubic diamond and sometimes
found in meteorites. As is taught in Science, volume 155, page 995 (1967)
by R. E. Hanneman, et al., Journal of Material Science, volume 22, page
3615 (1987) by T. Sekine, et al. and elsewhere, hexagonal diamond is
synthetically obtained in the form of a powder by the shock-wave
pressurization method, in which intense heat and extremely high pressure
are applied simultaneously to a powder of graphite, but this method is
absolutely not suitable for the purpose of depositing a coating film of
hexagonal diamond on the surface of a substrate. No conditions of the
plasma jet method are known under which the deposited coating film of
diamond has a predominantly hexagonal crystalline structure and the
crystallite size thereof could be small enough.
SUMMARY OF THE INVENTION
The present invention accordingly has an object to provide an improvement
in the method for the deposition of a coating film of diamond by using a
plasma jet blown at the surface of a substrate, by which the film of
diamond deposited on the surface could have a hexagonal crystalline
structure and the crystallite size thereof could be as small as 0.1 .mu.m
or even smaller without decreasing the efficiency of deposition.
Thus, the improvement of the present invention comprises, in a plasma jet
method for the deposition of a coating film of diamond having an at least
partly hexagonal crystalline structure by blowing a plasma jet of a
plasma-generating gas containing a gaseous carbon source compound at the
surface of a substrate in a gaseous atmosphere of a reduced pressure:
(a) using hydrogen alone as the plasma-generating gas;
(b) keeping the pressure of the gaseous atmosphere in the range from 1 Torr
to 300 Torr;
(c) keeping the surface of the substrate at a temperature in the range of
from 800.degree. C. to 1200.degree. C.; and
(d) making a temperature gradient of at least 13000.degree. C./cm within
the boundary layer of the gaseous atmosphere in the vicinity of and in the
direction perpendicular to the substrate surface.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1, 2 and 3 each show a part of the X-ray diffraction diagrams showing
the characteristic peaks from the cubic and hexagonal phases of diamond.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As is described above, the improvement provided by the invention comprises
four requirements (a) to (d) and the coating film of diamond deposited on
the substrate surface by the plasma jet method may have an at least partly
hexagonal crystalline structure only when these requirements are
satisfied. Namely, the scope of the invention comprises the atmospheric
conditions that hydrogen alone is used as the gas for plasma generation by
which a gaseous carbon source compound such as methane and the like is
pyrolyzed in the atmosphere kept under a pressure within a specified range
and the temperature conditions that the surface of the substrate is kept
at a temperature in a specified range and the temperature gradient in the
gaseous atmosphere exceeds a specified lower limit in the vicinity of or
within the boundary layer on and in the direction perpendicular to the
substrate surface.
The hydrogen plasma jet here implied is a jet stream of the plasma of
hydrogen gas ejected at a high velocity from the end opening of a nozzle
made from a metal such as copper as the anode with application of an
electric power between the anode and a cathode made from a refractory
metal of high melting point such as tungsten and molybdenum and inserted
into the anode nozzle under a continuous flow of hydrogen gas through the
nozzle. When a gaseous carbon source compound is introduced into the
plasma jet and the plasma jet is blown at the surface of a substrate, the
carbon source compound is decomposed in the plasma and deposited on the
substrate surface to form a film of diamond.
In the first place, the plasma-generating gas used in the invention is
hydrogen gas alone and does not contain any rare gas such as argon as in
the prior art methods. This requirement is based on the fact that hydrogen
gas has the largest coefficient of heat conductivity among gases so as to
contribute to the increase of the temperature gradient in the vicinity of
the substrate surface. When the plasma-generating gas is a mixture of
hydrogen and a rare gas such as argon, the temperature gradient in the
vicinity of the substrate surface cannot be large enough resulting in
predominance of the cubic crystalline structure over hexagonal in the
deposited film of diamond as compared with that obtained by using pure
hydrogen as the plasma-generating gas.
Secondly, the pressure of the gaseous atmosphere must be kept in the range
from 1 Torr to 300 Torr. Increase of the pressure over the above mentioned
upper limit approaching the normal pressure is also a factor which
decreases the temperature gradient in the atmosphere in the vicinity of
the substrate surface resulting in predominance of the cubic crystalline
structure of diamond.
Thirdly, it is essential that the surface of the substrate is kept at a
temperature in the range from 800.degree. to 1200.degree. C. This
condition can be achieved by mounting the substrate on a water-cooled
substrate holder. When the temperature of the substrate surface is too
high or, in particular, higher than 1400.degree. C., graphite is formed by
the pyrolysis of the carbon source compound so that the film of hexagonal
diamond deposited on the substrate surface would contain a considerable
amount of graphite and, by further increasing the temperature, the
hexagonal diamond once formed may eventually be converted into graphite.
The surface coating film of diamond deposited according to the inventive
method consists of very fine crystallites having a size of 0.1 .mu.m or
smaller. The film is formed of one or a mixture of two kinds or more of
the polytypes of hexagonal diamond having various structures including
those not yet identified. Accordingly, the surface of the coating film of
diamond deposited by the inventive method is very smooth as compared with
the coating film of cubic diamond deposited by a conventional thermal
plasma method which is also efficient in the growth rate of the film. The
above mentioned advantage is substantially not decreased unless the
fraction of the hexagonal diamond in the deposited film is 30% or smaller.
Since the diamond film deposited by the inventive method is a polycrystals
of polytypism, the Mohs hardness number of the film is not lower than that
alloted to a crystal of cubic diamond as the reference stone.
In the following, the method of the present invention is described in more
detail by way of examples.
EXAMPLE 1
A molybdenum-made disc having a thickness of 1 mm and a diameter of 50 mm
as a substrate was mounted on a water-cooled substrate holder in a plasma
chamber having an inner diameter of 150 mm and a height of 300 mm and a
copper-made 38 mm-long anode nozzle of a funnel-like form having a
diameter of 10 mm at the upper end and 4.5 mm at the untapered 26 mm-long
tubular lower portion was vertically held above the substrate at a
distance of 49 mm between the lower end opening of the anode nozzle and
the substrate surface. A tungsten-made rod-like cathode of 4 mm diameter
was inserted into the anode nozzle.
Hydrogen gas was introduced into the anode nozzle at a rate of 80 liters
(N.T.P.) per minute and methane as the carbon source gas was introduced as
a pulsating flow at a rate of 2 liters (N.T.P.) per minute through a pair
of oppositely facing horizontal small nozzles of 1.5 mm diameter held at a
height of 10 mm below the lower end opening of the anode nozzle. The
pressure inside the plasma chamber was kept at 180 Torr by the balance of
continuous evacuation and introduction of the gases.
When a direct-current voltage of 100 volts was applied with a discharge
current of 120 amperes between the anode nozzle and the cathode, hydrogen
plasma was generated and ejected as a jet out of the lower end opening of
the anode nozzle at the substrate surface. The surface of the substrate
was kept at a temperature of 1050.degree..+-.50.degree. C. The temperature
gradient within the boundary layer having a thickness of 0.19 mm on the
substrate surface was estimated to be about 23,000.degree. C./cm.
After 20 minutes of running in the above described manner, the substrate of
molybdenum disc taken out of the plasma chamber was examined to find that
a diamond film having a thickness of about 80 .mu.m was deposited on the
surface. The thus formed diamond film on the substrate surface was
evaluated by the scanning electron microscopy, X-ray diffractometry and
Raman spectroscopy. The results of the scanning electron microscopic study
indicated that the surface of the deposited diamond film was very smooth
as compared with the surface of a diamond film of the cubic crystalline
structure deposited by the conventional method and the crystallites had a
size of about 0.1 .mu.m or finer.
FIG. 1 of the accompanying drawing shows a part of the X-ray diffraction
diagrams of a diamond film having a cubic structure (peak 1) and the
diamond film obtained in this example (peak 2) determined by using
CuK.sub..alpha. line. The peak 1 at about 2.degree..theta.=44.degree. is
assignable to the (111) lattice plane of the cubic diamond. The peak 2, of
which the diffraction angle of the peak position is also about
2.degree..theta.=44.degree., seemingly indicated that the diamond film
obtained in the above described manner also had a cubic crystalline
structure with broadening of the peak width but it could be confirmed from
the results of the selected-area electron diffraction study that this peak
2 was a composite of several peaks characterizing hexagonal diamond and
representing the lattice planes of (11X), in which X is 0, 1, 2, 3, and so
on, the largest value of X depending on the kind of the polytype of
hexagonal diamond. The peaks corresponding to such lattice planes having a
lattice constant smaller or larger than (111) could be observed separately
from the peak for (111) when the conditions of the hydrogen plasma jet
method were modified as is shown by the curves 3 and 4 in FIGS. 2 and 3,
respectively.
In the Raman spectrum of the diamond film prepared in the above described
manner, two peaks at wave numbers of about 1140 cm.sup.-1 and 1470
cm.sup.-1, which are considered to be characteristic of hexagonal diamond,
were found in the spectrum while no peak was observed at about 1133
cm.sup.-1 which is characteristic of cubic diamond.
The result of the Mohs hardness test was that the film of hexagonal diamond
deposited on the substrate surface could form scratches on a crystal of
cubic diamond so that the Mohs hardness number thereof exceeded the
highest in the Mohs scale.
The above described analytical results of the deposited diamond film led to
a conclusion that the diamond film obtained in this example consisted of
96% of hexagonal diamond and 4% of cubic diamond.
EXAMPLE 2
The apparatus used in the hydrogen plasma jet deposition of a diamond film
on the molybdenum substrate was the same as in Example 1 except that the
distance between the substrate surface and the lower end opening of the
anode nozzle was 44 mm instead of 49 mm. The operating conditions for the
plasma jet deposition of the diamond film were also about the same as in
Example 1 except that the rate of methane introduction was 3.5 liters
(N.T.P.) per minute, the discharge voltage was 113 volts, the pressure
inside the plasma chamber was controlled at 400 Torr, the temperature of
the substrate surface was kept at 1040.degree..+-.50.degree. C. and the
temperature gradient within the boundary layer having a thickness of 0.51
mm on the substrate surface was estimated to be about 12,000.degree.
C./cm.
The substrate taken out of the plasma chamber after 20 minutes of running
in the above described manner was found to be coated with a film of
diamond having a thickness of 80 .mu.m. The analytical studies of this
diamond film undertaken in the same manner as in Example 1 gave a
conclusion that this diamond film consisted of cubic diamond only and the
surface of the film was coarse and rough as being composed of crystallites
of about 20 .mu.m size.
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